1. Field of the Invention
The present invention relates to an electromagnetic flowmeter of the type which has a measuring tube, electrodes formed on the measuring tube, a magnetic field generating means surrounding the electrodes and a case which encases the measuring tube, electrodes and the magnetic field generating means. More particularly, the present invention is concerned with an electromagnetic flowmeter which features a specific form of the measuring tube effective in improving resistance to thermal impact and a novel construction of the electrode portion which provides superior gas-tightness.
2. Description of the Related Art
An electromagnetic flowmeter is a device in which a magnetic field is perpendicularly applied to a conductive fluid flowing in a measuring tube so as to induce an electric current, whereby the flow rate of the fluid is measured by signals from electrodes which detect the induced electric current.
As discussed in a magazine MEASUREMENT AND CONTROL, Vol. 29, No. 9, pp. 27-34, a typical conventional electromagnetic flowmeter has a measuring tube which is made of a stainless steel or like material and which is lined at its inner side with an insulating lining material such as a natural rubber, synthetic rubber or a fluororesin. In recent years, measuring tubes are also broadly used which are made of non-conductive ceramics such as alumina (referring to as Al.sub.2 O.sub.3 hereinafter), in order to acquire greater levels of resistance to heat, corrosion and wear. In the known measuring tube made of a stainless steel, electrodes are secured to the measuring tube by inserting the electrodes into holes formed in the tube wall and then fixing them by means of an adhesive so as to obtain gas-tightness. When Al.sub.2 O.sub.3 is used as the tube material, electrodes associated with the tube are made of platinum which has sufficient resistance both to heat and corrosion at temperatures at which Al.sub.2 O.sub.3 are suitably sintered. Such electrodes can be secured in a gas-tight manner to the tube by, for example, a method which is disclosed in the specification of WO 83/02000 (PCT/EP82/00097), wherein holes of the same diameter as the electrodes are formed in the wall of Al.sub.2 O.sub.3 measuring tube before sintered, inserting the platinum electrodes into the holes and then sintering the Al.sub.2 O.sub.3 tube and the electrodes thereby integrating them.
These known electromagnetic flowmeter were not designed with sufficient consideration of the necessity for reducing a temperature difference which is established by the flowing fluid during the measurement between the inner and outer surfaces of the measuring tube. Consequently, a risk has been encountered that the ceramics measuring tube may be cracked due to application of excessively large thermal stress. Ceramics measuring tubes, therefore, could not be used in plants in which large thermal stress is caused on the measuring tube. For the same reason, use of ceramics measuring tubes has been restrained from being used in such fields that handle deleterious matters.
In case of the measuring tubes made of stainless steel, it has been difficult to maintain gas-tightness for a long time because the electrodes are secured to the measuring tube simply by an adhesive. On the other hand, in the case of a ceramics measuring tube in which the electrodes are held by shrink fit due to shrinkage of tube after the sintering, unduly large thermal stress is generated in the portions of the ceramics measuring tube holding the electrodes, with the result that the strength of the ceramics measuring tube is impaired. Furthermore, any defect in the boundary between the measuring tube and the electrode causes a reduction in the gas-tightness. It is also to be noted that the ceramics measuring tube cannot stand for use in an environment where heat cycles are applied to the tube, because repetition of heat cycles undesirably reduces the shrink fit force with which the electrodes are held, with the result that the gas-tightness is lost before long.